1. Field of the Invention
The present invention relates to an ionic compound, and more specifically, to an ionic compound suitable as a material for an ionic conductor of which an electrochemical device is constituted, and an electrolyte material, an electrolytic solution, and an electrolytic capacitor each containing the ionic compound.
2. Description of the Related Art
Each of ionic compounds uses an ionic substance formed of a compound constituted of a cation and an anion as an essential ingredient, and finds use in a variety of applications. Of the ionic compounds, an ionic compound having ionic conductivity is suitably used as an ion conductive material. To be specific, the ionic compound is suitably used as a component of an ionic conductor used as an essential ingredient in, for example, any one of various cells based on ion conduction. The component of the ionic conductor can function as at least one of an electrolyte and a solvent in an electrolytic solution of which the ionic conductor is constituted. In addition, the component of the ionic conductor can function as a solid electrolyte. Examples of the applications of the component of the ionic conductor include: cells having charging and discharging mechanisms such as a primary cell, a lithium (ion) secondary cell, and a fuel cell; and electrochemical devices such as an electrolytic capacitor, an electric double layer capacitor, a solar cell, and an electrochromic display device. In each of those electrochemical devices, in general, a cell is constituted of a pair of electrodes and an ionic conductor filling a gap between the electrodes.
Currently used as the ionic conductors are electrolytic solutions prepared by dissolving an electrolyte such as lithium perchlorate, LiPF6, LiBF4, tetraethylammonium fluoroborate, or tetramethylammonium phthalate, in an organic solvent such as γ-butyrolactone, N,N-dimethylformamide, propylene carbonate, or tetrahydrofuran. When dissolved in the organic solvent, the electrolyte dissociates into a cation and an anion to allow ionic conduction through the electrolytic solution. Solid electrolytes which allow ionic conduction in a solid state may also be used as an ionic conductor.
FIG. 1 shows a schematic cross sectional view of an embodiment of a conventional lithium (ion) secondary battery. The a lithium (ion) secondary battery has a positive electrode and a negative electrode each formed of an active substance, and an electrolytic solution constituted of an organic solvent and a lithium salt such as LiPF6 dissolved as a solute in the solvent, forms an ionic conductor between the positive and negative electrodes. During charging, the reaction C6Li→6C+Li+e− occurs on the negative electrode, the electron (e−) generated on the negative electrode surface migrates through the electrolytic solution to the positive electrode surface in the manner of ionic conduction. On the positive electrode surface, the reaction CoO2+Li+e−→LiCoO2 occurs and an electric current flows from the negative electrode to the positive electrode. During discharging, reverse reactions of those during the charging occur, and an electric current flows from the positive electrode to the negative electrode.
However, the an electrolytic solution forming an electrochemical device has the following problems: the organic solvent may readily volatilize and has a low flash point; liquid leakage may readily occur, resulting in lack in long-term reliability; and the electrolytic solution coagulates at low temperatures and therefore fails to exhibit performances as an electrolytic solution. Thus, there has been a demand for materials capable of improving those problems.
It has been disclosed that an ionic compound containing a sufficient number of anion portions (the number is not less than one) bonded to at least one cation portion M for securing entire electrical neutrality in which M represents hydroxonium, nitrosonium NO+, ammonium NH4+, a metal cation having a valence of m, an organic cation having a valence of m, or an organometallic cation having a valence of m, and the anion portions each correspond to one of the formulae RD—Y—C(C≡N)2− and Z-C(C≡N)2− can be used as an ion conductive material (see, for example, Japanese Patent Translation Publication No. 2000-508676 (p. 2 to 13 and 39 to 67)). Each of the anion portions is of a five-membered ring shape, or is a derivative of tetraazapentalene, and the derivatives of triazole, imidazole, and cyclopentadiene are described as examples of the anion portions in examples. To be specific, a tricyanomethide has been disclosed. However, there still remains a need for making contrivance in order that the compound may be a suitable material of which an electrolytic solution exerting excellent basic performance is constituted.
An electrolytic solution for driving an electrolytic capacitor prepared by dissolving a carboxylate of any one of the pentaalkylguanidines as a solute in γ-butyrolactone as an organic polar solvent has been disclosed (see, for example, Japanese Patent Application Laid-open No. Hei 9-97749 (p. 1 and 2)). However, the electric conductivity of a tertiary ammonium salt-based electrolytic solution is not sufficient as compared to that of a quaternary ammonium salt-based electrolytic solution. Accordingly, there still remains a need for making contrivance in order that an electrolytic solution capable of serving as a highly reliable electrolyte and having an additionally high electric conductivity may be developed.
An electrolytic solution for use in an electrochemical device such as a lithium secondary cell, an electrolytic capacitor, or an electric double layer capacitor is desirably excellent in an ionic conductivity and electrochemically stable at high electric potentials. To be specific, a solution prepared by dissolving a salt of, for example, triethylamine and maleic acid or phthalic acid (tertiary salt) in a solvent such as γ-butyrolactone, N,N-dimethylformamide, or ethylene glycol has been used as an electrolytic solution for an electrolytic capacitor (see, for example, Japanese Patent Application Laid-open No. Sho 54-7564). However, when a tertiary salt is used as described above, the following problem arises: the resultant solution has a low electric conductivity.
In view of the foregoing, a solution prepared by dissolving a salt of a tetraalkylammonium and, for example, maleic acid or phthalic acid (quaternary salt) in a solvent has been used (see, for example, Japanese Patent Application Laid-open No. Sho 62-264615). An electric conductivity in the case where a quaternary salt is used is higher than that in the case where a tertiary salt is used, but the case where a quaternary salt is used involves the following problem: a pH in a system increases. As a result, the following problem arises: sealing rubber for a capacitor deteriorates, and a liquid leaks from a cathode portion to adhere to the surface of a printed circuit board, thereby causing deficiencies such as corrosion and disconnection. In addition, the use of a quaternary salt increases an electric conductivity as compared to that in the case where a tertiary salt is used, but an additional increase in an electric conductivity is desired.